HV Wound Rotor Motor Characteristics

[skippyaust]

Am replacing 2 x 5MW 11kV slip-ring mill motors which are 10 years old but because they operate in desert conditions (very low humidity) exhibit very high brush wear and the carbon dust degrades the insulation of stators and rotors. They have been costly and unreliable to maintain and recently a rotor failed. Am looking for feedback on experience with BRUSH LIFTING GEAR and the pros and cons of STAR OR DELTA CONNECTED ROTORS FOR THE REPLACEMENT MOTORS

Thanks

 

[jbartos]

Suggestion: Visit

http://www.joliet-equipment.com/wound_rotor_ac_motors.htm

etc. for the motor details and options
The wye connected rotor is more frequent since:
1. The winding is simpler
2. Electrical stresses are smaller
3. Rotor controller is simpler
4. Etc.

 

[rbulsara]

If you really want to get rid of slip rings, seriously consider a MV autotransformer type starter for the replacement motors, a first choice for MV motors.

Star-delta is a bad choice for MV motors and even for large LV motors.

Autotransformers with 50%, 65% and 85% taps provide a good range of choice to adjust the starting voltage and hence the torque and you always have a smooth start. It is a proven technology too.

You need to evaluate the starting torque characteristics of the load to available torque when choosing a proper reduced voltage starter.

 

[Aquarius]

rbulsara

skippywest question has nothing to do with the star/delta starting, the Q is " the pros and cons of STAR OR DELTA CONNECTED ROTORS FOR THE REPLACEMENT MOTORS "

I now that:

Majority of rotors of SR motors are connected in star
Delat connection is probably cheeper
Has to do something with v/turn ?? ( need to do some calscs)

Note on the starting methods and installing MV transformer to get rid of the slip rings:

It is not as easy and may not be possible since the slip ring motors (SRM) are selected :

1 to start the load at the max motor torque by selecting suitable secondary resistors ( ohmic value)

2 large starting torque is obtain with a relatively low starting current (LRC = 1.5 to 2 FLC)

3 if starting frequency or load GD^2 is too large for the thermal resistivity of the ordinary s/cage motor

4 load requires cushion starting

Primarily SRM are selected to start a high inertia load or load that requires a v high starting torque across the full speed range. ( Secondary-for speed control)

Reducing voltage at start to reduce LRC will reduce the motor starting torque and extends s/time which is in conflict with the reason of selecting the SLM in a first place.

 

[rbulsara]

Aquarious:

Thank you for pointing out my oversight. Nonetheless, an autotranformer starter can get rid of any brush problems for "Replacement motors" provided starting torque requirement is met.

My two cents are:

1. Primary reason rotors are Star wound is to facilitate ease of inserting and taking out the resistors using only three slip rings. Inserting a resistor in a delta winding will at least require six slip rings and I would beleive only complicates the system besides adding maintenance and failure points. Therer may be other reasons too.

2. While your reasons for a SRM are correct, wound rotor is not necessarily used to get higher torque, but also often to have lower starting current for the a given torque! (technically both sentences are same, but serves different goals)

It is very likely that adeqaute torque is also available with a reduced voltage start.

Thanks

 

[Aquarius]

Thanks for comments bulsara , re. point 2 fully agree )

re. point 1 I have not heard of inserting resisitors in the rotor winding after a motor has been made and is in service, if that's what you mean ???.

On our sites we have both, star and delta connected rotors, all in the range of 2.5 to 6.5 MW 11kV, all these motors have rotors with 3 leads... After a motor is made, resistors can only be inserted in the outside circuit.

On the subject star/delta rotor connected windings :

Most SRM have rotors in star;
adventage - obvious one,lower insulation V required, disadvantage 3rd harmonics currents are not eliminated, in addition to being a pollution such current can activate the protection device

Delta - adventage - eliminate 3rd & its order harmonics current, disadvantage - circulating currents and consequently thermal lossess, for that reason most large motors have rotor windings connected in star.

Regards

 

[jbartos]

Suggestion/Question to the previous posting: Have you measured third harmonic values in the motor rotor? How large is it?
Normally, motors exhibit 5th harmonics, etc. but not much triplen harmonics.